(1) Field of the Invention
The present invention relates to a process for the preparation of an grain-oriented electrical steel sheet having excellent magnetic and film characteristics. More particularly, the present invention relates to a process in which the temperature for heating an electrical steel slab is lowered and the productivity is increased.
(2) Description of the Prior Art
A grain-oriented electrical steel sheet is used mainly as a core material for transformers, generators and other electric appliances, and this electrical sheet must have not only good magnetic characteristics such as good exciting and watt loss characteristics but also good film characteristics.
The grain-oriented electrical steel sheet is obtained by growing crystal grains having a Goss structure, having the {110} plane in the rolled plane and the &lt;001&gt; axis in the rolling direction, by utilizing the phenomenon of secondary recrystallization.
As is well known, the phenomenon of secondary recrystallization occurs during the finish annealing step, and to manifest a good secondary recrystallization, a fine precipitate of an inhibitor such as AlN, MnS or MnSe, which inhibits the growth of primary recrystallization grains at temperatures below the secondary recrystallization-manifesting temperature region during the finish annealing step, must be present in the steel. Accordingly, an electrical steel slab is heated at a temperature as high as 1350.degree. to 1400.degree. C. to completely solid-dissolve inhibitor-forming elements such as Al, Mn, S, Se and N, and the inhibitor elements completely solid-dissolved in the steel are finely precipitated in the form of AlN, MnS or MnSe by annealing in the hot-rolled sheet or in the stage of the intermediate thickness before the final cold rolling.
If this process is adopted, since the electrical steel slab is heated at a high temperature as mentioned above, the frequency of repairs necessary for the heating furnace is increased, resulting in increase of the maintenance costs, a reduction of the operating efficiency of the equipment, and an increase of the fuel unit.
To solve this problem, research and investigation have been made into a process for preparing a grain-oriented electrical steel sheet while lowering the temperature for heating an electrical steel slab.
For example, Japanese Unexamined Patent Publication No. 52-24116 proposes a preparation process in which the temperature for heating an electrical steel slab is lowered to 1100.degree. to 1260.degree. C. by incorporating a nitride-forming element, such as Al and Zr, Ti, B, Nb, Ta, V, Cr or Mo, in the steel.
Furthermore, Japanese Unexamined Patent Publication No. 59-190324 proposes a process in which the C content is kept below 0.01%, an electrical steel slab in which S, Se, Al and B are selectively included is used as the starting material, and pulsation annealing is effected by repeatedly heating the surface of the steel sheet at a high temperature for a short time at the primary recrystallization annealing conducted after the cold rolling, whereby the temperature for heating the electrical slab can be made lower than 1300.degree. C.
Moreover, Japanese Examined Patent Publication No. 61-60896 proposes a preparation process in which an electrical steel slab wherein the Mn content is adjusted to 0.08 to 0.45% and the S content is kept below 0.007% to lower the [Mn][S] product, and Al, P and N are incorporated, is used as the staring material, whereby the temperature for heating the slab can be made lower than 1280.degree. C.
Where grain-oriented electrical steel sheets are prepared according to these prior art techniques, however, defects such as "frosting" and "bear spots" are often formed on glass films of the final products.